Intro to Classes

Objectives

Understand difference between objects and classes
Understand instance variables and instance methods
Understand class variables and class methods

Class Definition of a person

Let's create our first class.

person.rb

class Person

end

This defines a class definition of a Person. The class keyword denotes the begining of a class definition.

To create a new instance of our class we write the following:

Person.new

A class is an imprint of a thing we want to create.

A particular instance of a class is a called an object. In general, languages that use objects as a primary means of data abstraction are said to be Object Oriented Programming (OOP) languages.

Interfaces

With classes we are also creating interfaces (or methods / functions) that anyone using the class can call in order to do certain things.

Much like a library, when you create a set of methods within your class you are defining the way the person using that class will interact with it.

It doesn't matter how the code in your class works, just that the interface gives methods to call and gives access to the relevant data.

We understand what this code would mean, even though we don't know how it would work:

person = Person.new("john")

person.birthday= "1/2/33"

puts person.calculate_age

Objects

Initialize and instance variables

In our class definition we can make use of an initialize method, which is run when a new instance of the class is created.

class Person
  def initialize
    puts "A new person was created"
  end
end

We can also make use of instance variables that are defined for each particular object and are available throughout other methods in the object. These variables are prefixed by an @ symbol, i.e. @my_var.

class Person

  def initialize(name)
    @name = name
  end

  def greet
    puts "Hello! My name is #{@name}."
  end
end

Now, when we create a new Person we are required to specify the name of the person.

person = Person.new("John")
person.greet

# => Hello! My name is John.

We can also use initialize to create a class instance with default values

class Person

  def initialize
    @age = 0
  end

  def age
    puts @age
  end
end

Our first goal is to duplicate the functionality of a data structure that holds data, like a javascript object (or a ruby hash).

So:

const person = {
  name: "Susan Chan",
  weight: 123
};

console.log( person.name );  // get value out

person.name = "John"; // change a value

With our Person class, where we only hold one piece of data, the minimum code we need would look like this:

class Person

  def initialize( age )
    @age = age
  end

  def get_age
    return @age
  end

  def set_age( new_age )
    @age = new_age
  end
end

Using Classes

Now, we can create instances of a person, and they act just like any other kind of data.

person1 = Person.new( 22 )
person2 = Person.new( 12 )
person3 = Person.new( 52 )
person4 = Person.new( 42 )
person5 = Person.new( 32 )

Or:

people = [
  Person.new( 22 ),
  Person.new( 12 ),
  Person.new( 52 ),
  Person.new( 42 ),
  Person.new( 32 )
]

Or these class instances can be in a hash, or given as data and stored inside of other class instances.

class Car
  def initialize( person )
    @owner = person
  end
end

person1 = Person.new( 32 )

car1 = Car.new( person1 )

Pairing Exercise

(ruby for beginners)

Ruby Classes